Recently, with an increase of a network transmission rate, optical fiber communication systems have been spreading rapidly.
As general characteristics of an optical fiber, propagation velocity of light differs depending on its wavelength, and the propagation velocity of a shorter wavelength is usually faster.
Accordingly, the propagation velocity becomes lower as the wavelength becomes longer so that the receiving time at a receiving apparatus varies owing to slight difference in the wavelength.
The delay difference (wavelength dispersion) of light due to wavelength will bring about a waveform distortion and degrade a bit error characteristic.
As for the effect of the waveform distortion due to wavelength dispersion, although it is small in an optical transmission system with a comparatively low transmission rate, it increases in a recent high rate optical transmission system because the dispersion tolerance reduces with an increase of the bit rate.
In particular, at a transmission rate not less than 40 Gb/s, since the dispersion tolerance is low and deterioration of transmission performance occurs owing to slight changes in the dispersion amount due to changes in temperature, a high-accuracy dispersion compensation technique is necessary.
As a dispersion compensation method, there is a method of employing a dispersion compensating fiber with the same dispersion amount of the opposite sign so as to cancel out the dispersion amount of a transmission line, or a method of using a variable dispersion compensator capable of controlling the dispersion amount of a transmission line.
In the high-speed communication not less than 40 Gb/s, since it requires highly-accurate dispersion compensation, a variable dispersion compensator is generally used.
As for a place where a dispersion compensator is used, both the receiving apparatus side and transmission apparatus side are conceived.
When using the dispersion compensator at the receiving apparatus side, the received signal is decoded by providing a received distorted waveform with the dispersion amount of the opposite sign.
On the other hand, when using the dispersion compensator at the transmission apparatus side, the opposite sign dispersion amount is added in advance so as to cancel out the residual dispersion amount of the transmission line, thereby enabling the receiving apparatus side to receive a right signal (the method is referred to as a “dispersion pre-equalization method” from now on).
In an optical transmission system employing the dispersion pre-equalization method as shown in FIG. 6, a dispersion compensator installed in an optical communication apparatus A on the transmission side controls the dispersion amount of a transmission line. To set the dispersion amount at the optimum value, it is necessary for an own-apparatus error rate monitoring unit installed in a receiving side optical communication apparatus B to monitor an error rate (signal error rate), and for the transmission side optical communication apparatus A to collect monitoring results (see Patent Document 1, for example).